ARM: dts: msm: disable sdhc_2 for QRD8939 SKUL

Disable sdhc_2 as hw doesn't support SD card.

Change-Id: Id5a6f59ea3887d1f8980228650012a48b6833fe3
Signed-off-by: Guoping Yu <guopingy@codeaurora.org>
Signed-off-by: Changmin Liu <changmil@codeaurora.org>